In a cochlear implant system with a total of M channels, the stimulation rate (i.e., the number of pulses per second that may be applied to a user of the cochlear implant system) on each individual channel can determine the amount of temporal information transferred over that channel. High stimulation rates can be good for transferring more temporal information to the user. However, stimulation rates beyond a certain maximum threshold may not result in the transfer of more temporal information and may actually waste power.
The loudness of one particular pulse is dependent on the amount of charge delivered, i.e., the pulse width or duration (measured in seconds) times the pulse amplitude (measured in amps). Therefore, to deliver sound at a most comfortable loudness level (“M level”) associated with the user, with enough temporal information to support optimal sound quality, a cochlear implant may deliver high amplitude, short duration, pulses, which in turn supports high stimulation rates.
Unfortunately, generating high amplitude pulses requires a relatively high amount of power. To minimize power consumption, the cochlear implant may instead deliver low amplitude, long duration, pulses. While increasing the pulse duration may minimize power consumption, it may disadvantageously decrease the stimulation rate, which in turn may reduce the amount of temporal information that is transferred to the user.
One way to overcome this limitation is to use an N-of-M channel selection strategy in which a cochlear implant system only stimulates a subset of the available electrodes in each given stimulation frame. In an N-of-M channel selection strategy, a cochlear implant system divides an incoming audio signal into M analysis channels (or simply “channels”) and then presents only N analysis channels to the user (e.g., by applying electrical stimulation representative of the signals contained within the N analysis channels by way of a plurality of intracochlear electrodes).
Because a number of channels are not selected (i.e., dropped) during each stimulation frame while a cochlear implant system operates in accordance with an N-of M channel selection strategy, the N-of M channel selection strategy may allow for a relatively high stimulation rate while at the same time minimizing power consumption. However, dropping channels may potentially result in some spectral information not being presented to the user, which in turn may lead to poor cochlear implant system performance. Thus, there is a tradeoff between preserving temporal information by keeping the stimulation rate high and preserving spectral information by keeping the value of N high.
Typically, N is arbitrarily set in a stimulation program, meaning that all cochlear implant users of that stimulation program will have the same number of channels selected for presentation during each stimulation frame. However, because each user has different M levels and pulse durations, fixing N across users can result in different stimulation rates for the users. For example, a fixed N for one user may result in a very low stimulation rate, which may lead to a loss of temporal information. This user may actually perform better with a lower N and a higher stimulation rate. For the same fixed N, another user may have a very high stimulation rate, beyond that needed to support good temporal information transfer. This user may actually benefit from having a higher N with improved spectral information transfer and a slightly lower stimulation rate that is still sufficient to support good temporal information transfer.